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Re: _Alignof structs

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Thiago Adams <thiago.adams@gmail.com> writes:
> On Thursday, November 10, 2022 at 4:57:12 PM UTC-3, Keith Thompson wrote:
>> Thiago Adams <thiago...@gmail.com> writes: 
>> > What is the idea behind alignof structs? 
>> > 
>> > I tried this code in gcc 
>> > 
>> > struct X { 
>> > char i; 
>> > char s; 
>> > }; 
>> > _Static_assert(_Alignof(struct X) == 1, ""); 
>> > 
>> > int main(){} 
>> > 
>> > The result seems to be the alignof the first element of struct. 
>> > Changing i for int or double changes the ouput. 
>> > I was expecting the max alignment before checking it.
>> I'm not sure why you'd expect the max alignment. 
>
> I  was expecting a fixed the max alignment to be possible to 
> have a fixed reference for padding. The way I think it works (with padding) 
> is to have each member positioned on it own alignment. 
> I am trying to see now (thinking in some samples), if placing struct at the
> max member aligment will result in each member on its own aligment (considering padding)

I don't understand.

For this structure, no padding is necessary, and the members can be
accessed correctly regardless of the alignment of the structure.  The
maximum alignment of any member is 1 (char is 1 byte, and char arrays
wouldn't work if char required a stricter alignment), which means that
the alignment of the struct has to be *at least* 1.

What exactly do you mean by "max alignment", and why are you expecting
it to be something other than 1?

I wouldn't have been surprised of the alignment of the structure had
been 2, but gcc chooses to give it a 1-byte alignment, likely because
the relevant ABI requires it.

-- 
Keith Thompson (The_Other_Keith) Keith.S.Thompson+u@gmail.com
Working, but not speaking, for XCOM Labs
void Void(void) { Void(); } /* The recursive call of the void */

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_Alignof structs Thiago Adams <thiago.adams@gmail.com> - 2022-11-10 10:50 -0800
  Re: _Alignof structs Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2022-11-10 11:56 -0800
    Re: _Alignof structs Thiago Adams <thiago.adams@gmail.com> - 2022-11-11 02:57 -0800
      Re: _Alignof structs Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2022-11-11 15:07 -0800
        Re: _Alignof structs Thiago Adams <thiago.adams@gmail.com> - 2022-11-11 18:08 -0800
  Re: _Alignof structs Richard Damon <Richard@Damon-Family.org> - 2022-11-11 22:09 -0500
  Re: _Alignof structs Andrey Tarasevich <andreytarasevich@hotmail.com> - 2022-11-11 19:22 -0800
    Re: _Alignof structs Thiago Adams <thiago.adams@gmail.com> - 2022-11-12 02:53 -0800
      Re: _Alignof structs Po Lu <luangruo@yahoo.com> - 2022-11-14 10:28 +0800
        Re: _Alignof structs Ben Bacarisse <ben.usenet@bsb.me.uk> - 2022-11-14 02:44 +0000
          Re: _Alignof structs "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2022-11-13 18:44 -0800

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